This invention relates to an optical disk reproducing apparatus for reproducing information recorded on an optical disk, such as a CD (compact disk) or an LVD (laser vision disk), and more particularly, to a system for avoiding a so-called track stray upon effecting a track jump.
In an optical disk reproducing apparatus, the tracing position is often forcibly moved from one track to another. This movement of the tracing position is generally called a track jump. A track jump is effected, for example, when a user of a CD player is trying to locate the beginning of a piece of music in a CD player. The track jump is of two types: a one-track jump is effected by supplying a jump pulse to the tracking actuator; and a multi-track jump is effected by moving the optical pickup itself by means of a slider. In the case of a one-track jump, the jump pulse is composed of a pulse for moving the focusing position of the objective lens and a brake pulse for diminishing the acceleration effected by the above-mentioned pulse.
A problem with the track jump is a phenomenon called a "tracking stray". This is a phenomenon in which the actuator fails to stop at the destination track and instead moves onto some other tracks. A tracking stray occurs when the setting of the tracking actuator after a track jump is poorly made. Conventional optical disk reproducing apparatuses have been equipped with a circuit called a "brake circuit" for avoiding such a track stray. The brake circuit is a circuit for improving the setting of the actuator which drives the pickup when effecting a track jump. The brake circuit is designed to operate from the start of a track jump to the termination thereof, at predetermined time intervals.
The operation of this brake circuit will now be described in detail.
As disclosed in Japanese Patent Laid-Open No. 57-120276, the brake circuit utilizes the fact that the phase relationship between a signal obtained by extracting the envelope of a RF (radio frequency) signal reproduced by the pickup (hereinafter referred to as the "envelope signal") and a tracking error signal differs depending on the direction in which the light spot for signal reading is moved. The above-mentioned phase relationship when the light spot for signal reading moves radially across a track from the inside to the outside is shifted by 90.degree. with respect to the phase relationship when the light spot moves across the track from the outside to the inside.
The operation of the brake circuit is as follows: in the area where the tracking servo loop operates as a negative feedback loop, the tracking servo loop is set in a closed state, and, in the area where the tracking servo loop operates as a positive feedback loop, the tracking servo loop is set in an open state. More specifically, when, during a track jump operation, the light spot for signal reading moves from the center of one track and passes across the line which is substantially at the center of an inter-track section (section between two adjacent tracks), the tracking servo loop is set in the closed state, and, when the light spot for signal reading moves from the line which is substantially at the center of an inter-track section and passes across the center of a track, the tracking servo loop is set in the open state. This helps to diminish the relative speed of the light spot for signal reading with respect to the optical disk each time this light spot moves across a track.
In other words, the actuator which drives the pickup is braked each time the light spot for signal reading moves across a track. Accordingly, the setting of the actuator is improved, thereby enabling it to jump to a desired track.
A conventional optical disk reproducing apparatus, however, has the following problems: If there is some defect in the target or object track to which the pickup is to be jumped, the RF signal is not emitted in the normal manner, with the result that the comparison signal is not emitted in the normal manner, either. Thus, there is a problem that the brake circuit might set the tracking servo loop in an open state, when it should instead be set in the closed state, or vice versa, thereby causing a track stray.